1. Field of the Invention
The present invention relates to a composition for a ceramic substrate and a ceramic circuit component, and more particularly to a composition for a ceramic substrate which can be sintered at a temperature as low as 1,000xc2x0 C. or less, and a ceramic circuit component, such as a multilayered integrated circuit component and a thick-film hybrid circuit component, which is fabricated using the composition for the ceramic substrate.
2. Description of the Related Art
Currently, alumina substrates are predominantly used as ceramic substrates. In order to obtain an alumina substrate, firing must be performed at a temperature as high as approximately 1,600xc2x0 C., and thereby, for example, when a multilayered circuit component is fabricated using the alumina substrate, a metal having a high melting point must be used for internal conductors. However, the metal having a high melting point generally has a high resistance and is thus unsuitable for conductors used for multilayered circuit components in which higher frequencies and higher speeds are increasingly in demand.
Accordingly, glass-ceramic substrates having a firing temperature of 1,000xc2x0 C. or less, which enable use of metals having a low resistance as internal conductors, such as Au, Ag, Agxe2x80x94Pd, Agxe2x80x94Pt and Cu, have been receiving attention and various types of glass-ceramic substrates have been developed.
For example, Japanese Examined Patent Publication No. 3-53269 discloses a low-temperature sinterable ceramic substrate which is obtained by mixing 50% to 64% by weight of powdered glass and 50% to 35% by weight of powdered Al2O3, followed by sintering at 800 to 1,000xc2x0 C.
However, with respect to such a substrate in which glass is loaded at the rate of 50% or more, the proportion of crystalline substances in the sintered substrate is decreased, and thus the dielectric loss of the substrate may be increased or it may be difficult to obtain high mechanical strength, which is disadvantageous. Additionally, when a thick-film electrode and a thick-film resistor are baked on the surface of the sintered substrate, the substrate easily deforms under the influence of the remaining glass, which is also disadvantageous.
As a solution to the problems described above, the composition of the powdered glass as a starting material may be arranged so as to be easily crystallized after sintering, thus increasing the proportion of crystalline substances in the sintered substrate. However, with a substrate having a large proportion of glass in the starting material, such as with a glass load of 50% or more, strain in the substrate caused by the crystallization of glass during firing is influential, and deformation, such as warpage and cracking, easily occurs in the sintered substrate, which is disadvantageous.
Japanese Examined Patent Publication No. 4-42349 discloses a low-temperature sinterable ceramic composition in which 40% to 50% by weight of powdered glass, composed of 10% to 55% by weight of MO (where M is at least one of Ca and Mg), 0% to 30% by weight of Al2O3, 45% to 70% by weight of SiO2, and 0% to 30% by weight of B2O3, and 60% to 50% by weight of powdered Al2O3 are mixed and fired at 1,100xc2x0 C. or less. The above patent publication also discloses that by increasing the proportion of the powdered Al2O3 in the starting material, a transverse strength of 2,600 to 3,200 kgf/cm2 (255 to 314 MPa) can be obtained. However, such transverse strength is lower than the transverse strength (approximately 350 MPa) of the alumina substrate which is used as a circuit substrate, and higher strength is desired.
The substrate disclosed in the same patent publication has a coefficient of thermal expansion in the range from 4.0 to 5.7 ppm/xc2x0 C. It has been believed that a substrate having a low coefficient of thermal expansion is preferable on the assumption that a silicon chip (IC) having a coefficient of thermal expansion of 3.5 ppm/xc2x0 C. is directly mounted on the substrate. However, due to the development of a mounting method in which stress is relieved using a cushioning material such as an underfilling, a mismatch in the coefficient of thermal expansion between the ceramic substrate and the silicon chip does not greatly cause a problem. In addition, the size of the silicon chip has not increased as has been expected.
Under the circumstances where the ceramic substrate is joined to a larger resin substrate as a motherboard, a mismatch in the coefficient of thermal expansion between the ceramic substrate and the resin substrate is rather influential. For example, a typical glass epoxy (FR-4) has a coefficient of thermal expansion of 14 to 16 ppm/xc2x0 C., and an epoxy reinforced with Aramid fibers has a coefficient of thermal expansion of approximately 8 ppm/xc2x0 C. If a degree of mismatch in the coefficient of thermal expansion between the ceramic substrate and the resin substrate is increased, the reliability of the connection between the two substrates is lost, which is disadvantageous.
To overcome the above described problems, preferred embodiments of the present invention provide a composition for a ceramic substrate and a ceramic circuit component fabricated using the same, in which the problems described above can be solved.
More specifically, in accordance with the preferred embodiments of the present invention, a composition for a ceramic substrate used for electronic circuits with a firing temperature of 1,000xc2x0 C. or less is provided, thus enabling metals having a low resistance, such as Au, Ag, Agxe2x80x94Pd, Agxe2x80x94Pt and Cu, to be used as internal conductors. In a ceramic substrate obtained by firing the composition, it is possible to achieve a transverse strength of 300 MPa or more, a Q factor (1 MHZ) of 1,400 or more, and a coefficient of thermal expansion of 6.0 ppm/xc2x0 C. or more.
One preferred embodiment of the present invention provides a composition for a ceramic substrate, comprising a mixture of: powdered borosilicate-based glass comprising about 5% to 17.5% by weight of B2O3, about 28% to 44% by weight of SiO2, 0% to about 20% by weight of Al2O3, and about 36% to 50% by weight of MO (where MO is at least one selected from the group consisting of CaO, MgO and BaO); and a powdered ceramic; in which the amount of the powdered borosilicate-based glass is about 40% to 49% by weight based on the total amount of the composition for a ceramic substrate, and the amount of the powdered ceramic is about 60% to 51% by weight based on the total amount of the composition for a ceramic substrate.
Preferably, the composition for the ceramic substrate has a coefficient of thermal expansion of about 6.0 ppm/xc2x0 C. or more after sintering.
Preferably, the powdered ceramic contains powdered alumina.
The borosilicate-based glass may contain at least one alkali metal oxide selected from the group consisting of Li2O, K2O and Na2O, the amount of the alkali metal oxide being about 5 parts by weight or less relative to 100 parts by weight of the total amount of the B2O3, SiO2, Al2O3, and MO.
The borosilicate-based glass may contain at least one compound selected from the group consisting of TiO2, ZrO2, Cr2O3, CaF2 and CuO, the amount of the compound being about 5 parts by weight or less relative to 100 parts by weight of the total amount of the B2O3, SiO2, Al2O3 and MO.
Another preferred embodiment of the present invention provides a ceramic circuit component comprising a substrate obtained by molding and sintering the composition for the ceramic substrate described above and a conductive circuit provided in association with the substrate.
In the ceramic circuit component, the conductive circuit preferably contains at least one metal selected from the group consisting of Ag, Au and Cu as a principal ingredient.
According to the composition for the ceramic substrate of the present invention as described above, since low-temperature sintering at 1,000xc2x0 C. or less is enabled, when a ceramic circuit component provided with a conductive circuit containing a metal having a low resistance, such as an Ag-based metal or a Cu-based metal is fabricated, firing can be performed simultaneously with the metal for the conductive circuit. In the ceramic substrate fabricated using the composition, it is possible to achieve a high mechanical strength, a low dielectric constant, a low loss and a high coefficient of thermal expansion required for the substrate. Accordingly, a ceramic circuit component, such as a multilayered ceramic circuit component, having satisfactory characteristics and high reliability can be obtained.
In particular, since it is possible to achieve in accordance with the composition for the ceramic substrate of the present invention, a coefficient of thermal expansion of about 6.0 ppm/xc2x0 C. or more, satisfactory matching in the coefficient of thermal expansion with a motherboard composed of, for example, an epoxy resin, is obtained, resulting in high connection reliability.
In accordance with the composition for the ceramic substrate of the present invention, when the borosilicate-based glass contains at least one alkali metal oxide selected from the group consisting of Li2O, K2O and Na2O with an amount of about 5 parts by weight or less relative to 100 parts by weight of the total of B2O3, SiO2, Al2O3 and MO, the softening and flow properties of the glass are accelerated, and even if the glass amount is reduced in the composition for the ceramic substrate, a relatively low sintering temperature can be maintained.
In accordance with the composition for the ceramic substrate of the present invention, when the borosilicate-based glass contains at least one compound selected from the group consisting of TiO2, ZrO2, Cr2O3, CaF2 and CuO with an amount of about 5 parts by weight or less relative to 100 parts by weight of the total of B2O3, SiO2, Al2O3 and MO, the crystallization of the glass is accelerated, and it is possible to further improve the high mechanical strength and the low loss of the resultant ceramic substrate.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawing.